1. Field of the Invention
The present invention relates to a method and an apparatus for feeding a sheet-shaped recording material, and in particular relates to operation control of roller pairs for nipping and feeding the recording material.
2. Description of the Related Art
A photo printer for recording an image on a sheet-shaped photosensitive material is widely known. In this kind of the photo printer, recording light is applied in a scanning direction while the photosensitive material is fed in a sub-scanning direction perpendicular to the scanning direction. The photo printer includes a plurality of feeding roller pairs comprising a capstan roller and a nip roller. The recording material is nipped and fed by the feeding roller pair.
In order to prevent density unevenness of a recording image to be caused by fluctuation of a feeding speed, it is necessary to accurately feed the photosensitive material at the time of image recording. In view of this, Japanese Patent Laid-Open Publication No. 2001-33883 teaches nip rollers respectively disposed at an upstream side and a downstream side of a record head for irradiating the recording light. The nip roller is movable between a nip position where the photosensitive material is nipped, and a release position where the nip is released. The respective nip rollers start to move when an anterior end or a posterior end of the photosensitive material has reached a predetermined position. In virtue of this, nipping and releasing the photosensitive material are performed at prescribed timing even if a length and a feeding speed of the photosensitive material are different. Thus, quality of the recording image may be kept in a good condition.
Meanwhile, Japanese Patent Laid-Open Publication No. 2002-3002 teaches nip rollers of an upstream side and a down stream side, which are moved by a single pulse motor. A movement speed of the nip roller is changed in accordance with a length of the photosensitive material. By doing so, shock to be caused to the photosensitive material is reduced when a feeding roller pair performs nipping and releasing.
In order to improve processing ability of the photo printer, it is preferable to increase the feeding speed of the photosensitive material. In addition, it is preferable to reduce an interval between the photosensitive materials successively fed. In this case, there is a possibility that drive timing of the respective nip rollers overlap. That is, timing for moving the upstream nip roller to the release position occurs while the downstream nip roller moves toward the nip position.
The drive timing of the nip rollers are usually designed so as not to overlap with each other. In case the drive timing of the nip rollers overlap, it is necessary to temporarily stop feeding the photosensitive material, since an operation is judged as being abnormal. Alternatively, it is necessary to stop the operation itself of the photo printer. In this case, the feeding speed changes while the image is recorded. Thus, the density unevenness is most likely to be caused. Further, there arises a problem in that the processing ability of the photo printer remarkably deteriorates, since feeding the photosensitive material is stopped.